Elevated levels of oxidized cholesterol metabolites in Lewy body disease brains accelerate alpha-synuclein fibrilization

Oxidative stress, inflammation and alpha-synuclein overexpression confer risk for development of alpha-synucleinopathies-neurodegenerative diseases that include Parkinson disease and Lewy body dementia. Dopaminergic neurons undergo degeneration in these diseases and are particularly susceptible to oxidative stress because dopamine metabolism itself creates reactive oxygen species. Intraneuronal deposition of alpha-synuclein as amyloid fibrils or Lewy bodies is the hallmark of these diseases. Herein, we demonstrate that concentrations of oxidative cholesterol metabolites derived from reactive oxygen species are elevated in the cortices of individuals with Lewy body dementia relative to those of age-matched controls, and we show that these metabolites accelerate alpha-synuclein aggregation in vitro. The increase in the production of these cytotoxic cholesterol metabolites is also observed in a dopaminergic cell line that overexpresses alpha-synuclein. By extension, these data lead to the hypothesis that oxidative stress produces cholesterol aldehydes that enable alpha-synuclein aggregation, leading to a pathologic cycle.     

Kinetic and structural analysis of the early oxidation products of dopamine: analysis of the interactions with alpha-synuclein

Oxidative stress appears to be directly involved in the pathogenesis of several neurodegenerative disorders, including Alzheimer and Parkinson diseases. Nigral dopaminergic neurons are particularly exposed to oxidative stress because a pathological accumulation of cytosolic dopamine gives rise to various toxic molecules, including free radicals and reactive quinones. These latter species can react with proteins preventing them from exerting their physiological functions. Among the possible targets of quinones, alpha-synuclein is of primary interest because of its direct involvement in dopamine metabolism. Contrary to the neurotoxic processes, neuromelanin synthesis seems to play a protective role by its ability to sequester a variety of potentially damaging substances. In this study, we carried out a kinetic and structural analysis of the early oxidation products of dopamine. Specifically, considering the potential high toxicity of aminochrome for both cells and mitochondria, we focused our attention on its rearrangement to 5,6-dihydroxyindole. After the spectroscopic characterization of the products derived from the oxidation of dopamine, the structural information obtained was used to analyze the reactivity of quinones toward alpha-synuclein. Our results suggest that indole-5,6-quinone, rather than dopamine-o-quinone or aminochrome, is the reactive species. We propose that the observed reactivity could represent a general reaction pathway whenever cysteinyl residues are absent in proteins or if they are sterically protected.     

Dopamine-dependent neurotoxicity of alpha-synuclein: a mechanism for selective neurodegeneration in Parkinson disease

The mechanism by which dopaminergic neurons are selectively lost in Parkinson disease (PD) is unknown. Here we show that accumulation of alpha-synuclein in cultured human dopaminergic neurons results in apoptosis that requires endogenous dopamine production and is mediated by reactive oxygen species. In contrast, alpha-synuclein is not toxic in non-dopaminergic human cortical neurons, but rather exhibits neuroprotective activity. Dopamine-dependent neurotoxicity is mediated by 54 83-kD soluble protein complexes that contain alpha-synuclein and 14-3-3 protein, which are elevated selectively in the substantia nigra in PD. Thus, accumulation of soluble alpha-synuclein protein complexes can render endogenous dopamine toxic, suggesting a potential mechanism for the selectivity of neuronal loss in PD.     

Effect of 4-hydroxy-2-nonenal modification on alpha-synuclein aggregation

Several observations have implicated oxidative stress and aggregation of the presynaptic protein alpha-synuclein in the pathogenesis of Parkinson disease. alpha-Synuclein has been shown to have affinity for unsaturated fatty acids and membranes enriched in polyunsaturated fatty acids, which are especially sensitive to oxidation under conditions of oxidative stress. One of the most important products of lipid oxidation is 4-hydroxy-2-nonenal (HNE), which has been implicated in the pathogenesis of Parkinson disease. Consequently, we investigated the effects of the interaction of HNE with alpha-synuclein. Incubation of HNE with alpha-synuclein at pH 7.4 and 37 degrees C resulted in covalent modification of the protein, with up to six HNE molecules incorporated as Michael addition products. Fourier transform infrared and CD spectra indicated that HNE modification of alpha-synuclein resulted in a major conformational change involving increased beta-sheet. HNE modification of alpha-synuclein led to inhibition of fibrillation in an HNE concentration-dependent manner. This inhibition of fibrillation was shown to be due to the formation of soluble oligomers based on size exclusion high pressure liquid chromatography and atomic force microscope data. Small angle x-ray scattering analysis indicated that the HNE-induced oligomers were compact and tightly packed. Treatment with guanidinium chloride demonstrated that the HNE-induced oligomers were very stable with an extremely slow rate of dissociation. Addition of 5 mum HNE-modified oligomers to primary mesencephalic cultures caused marked neurotoxicity because the integrity of dopaminergic and GABAergic neurons was reduced by 95 and 85%, respectively. Our observations indicate that HNE modification of alpha-synuclein prevents fibrillation but may result in toxic oligomers, which could therefore contribute to the demise of neurons subjected to oxidative damage.     

DJ-1 up-regulates glutathione synthesis during oxidative stress and inhibits A53T alpha-synuclein toxicity

DJ-1 is the third gene that has been linked to Parkinson disease. Mutations in the DJ-1 gene cause early onset PD with autosomal recessive inheritance. To clarify the mechanism of DJ-1 protection, we have overexpressed the gene in cultured dopaminergic cells that were then subjected to chemical stress. In the rat dopaminergic cell line, N27, and in primary dopamine neurons, overexpression of wild type DJ-1 protected cells from death induced by hydrogen peroxide and 6-hydroxydopamine. Overexpressing the L166P mutant DJ-1 had no protective effect. By contrast, knocking down endogenous DJ-1 with antisense DJ-1 rendered cells more susceptible to oxidative damage. We have found that DJ-1 improves survival by increasing cellular glutathione levels through an increase in the rate-limiting enzyme glutamate cysteine ligase. Blocking glutathione synthesis eliminated the beneficial effect of DJ-1. Protection could be restored by adding exogenous glutathione. Wild type DJ-1 reduced cellular reactive oxygen species and reduced the levels of protein oxidation caused by oxidative stress. By a separate mechanism, overexpressing wild type DJ-1 inhibited the protein aggregation and cytotoxicity usually caused by A53T human alpha-synuclein. Under these circumstances, DJ-1 increased the level of heat shock protein 70 but did not change the glutathione level. Our data indicate that DJ-1 protects dopaminergic neurons from oxidative stress through up-regulation of glutathione synthesis and from the toxic consequences of mutant humanalpha-synuclein through increased expression of heat shock protein 70. We conclude that DJ-1 has multiple specific mechanisms for protecting dopamine neurons from cell death.     

Alpha-synuclein overexpression and aggregation exacerbates impairment of mitochondrial functions by augmenting oxidative stress in human neuroblastoma cells

Overexpression of alpha-synuclein and oxidative stress has been implicated in the neuronal cell death in Parkinson's disease. Alpha-synuclein associates with mitochondria and excessive accumulation of alpha-synuclein causes impairment of mitochondrial functions. However, the mechanism of mitochondrial impairment caused by alpha-synuclein is not fully understood. We recently reported that alpha-synuclein associates with mitochondria and that overexpression of alpha-synuclein causes nitration of mitochondrial proteins and release of cytochrome c from the mitochondria [Parihar M.S., Parihar A., Fujita M., Hashimoto M., Ghafourifar P. Mitochondrial association of alpha-synuclein causes oxidative stress. Cell Mol Life Sci. 2008a;65:1272–1284]. The present study shows that overexpression of alpha-synuclein A53T or A30P mutants or wild-type in human neuroblastoma cells augmented aggregation of alpha-synuclein. Immunoblotting and immuno-gold electron transmission microscopy show localization of alpha-synuclein aggregates within the mitochondria of overexpressing cells. Overexpressing cells show increased mitochondrial reactive oxygen species, increased protein tyrosine nitration, decreased mitochondrial transmembrane potential, and hampered cellular respiration. These findings suggest an important role for mitochondria in cellular responses to alpha-synuclein.     

多巴胺能神经元线粒体的氧化损伤与帕金森病

帕金森病(Parkinson’s disease,PD)的一个主要病理特征就是中脑黑质多巴胺能神经元的丧失,目前研究认为该病理变化与多种因素有关,包括蛋白质异常积聚、泛素蛋白酶体系统功能异常、神经炎症、线粒体损伤和氧化应激。在帕金森病人和动物模型中,中脑黑质有着明显的氧化改变。帕金森病的遗传和环境因素均会作用于线粒体,尤其对线粒体呼吸链复合体I有着抑制作用,造成线粒体损伤,产生活性氧(ROS)。活性氧的大量产生造成脂类、蛋白质和DNA的氧化,从而加剧多巴胺能神经元的线粒体和细胞损伤。多巴胺代谢过程中会产生活性氧,该自身代谢特点决定了多巴胺能神经元存在有较高的氧化应激,易受环境因素的影响。因而,线粒体的氧化损伤在帕金森病病理发生中起着重要作用。     

Reactive Macrophages Increase Oxidative Stress and Alpha-Synuclein Nitration During Death of Dopaminergic Neuronal Cells in Co-Culture: Relevance to Parkinson’s Disease

Parkinson’s disease (PD) is characterized by progressive degeneration of dopaminergic neurons and a substantial decrease in the neurotransmitter dopamine in the nigro-striatal region of the brain. Increased markers of oxidative stress, activated microglias and elevated levels of pro-inflammatory cytokines have been identified in the brains of patients with PD. Although the precise mechanism of loss of neurons in PD remains unclear, these findings suggest that microglial activation may contribute directly to loss of dopaminergic neurons in PD patients. In the present study, we tested the hypothesis that activated microglia induces nitric oxide-dependent oxidative stress which subsequently causes death of dopaminergic neuronal cells in culture. We employed lipopolysaccharide (LPS) stimulated mouse macrophage cells (RAW 264.7) as a reactive microglial model and SH-SY5Y cells as a model for human dopaminergic neurons. LPS stimulation of macrophages led to increased production of nitric oxide in a time and dose dependent manner as well as subsequent generation of other reactive nitrogen species such as peroxynitrite anions. In co-culture conditions, reactive macrophages stimulated SH-SY5Y cell death characterized by increased peroxynitrite concentrations and nitration of alpha-synuclein within SH-SY5Y cells. Importantly 1400W, an inhibitor of the inducible nitric oxide synthase provided protection from cell death via decreasing the levels of nitrated alpha-synuclein. These results suggest that reactive microglias could induce oxidative stress in dopaminergic neurons and such oxidative stress may finally lead to nitration of alpha-synuclein and death of dopaminergic neurons in PD.     

Proteasome inhibition induces glutathione synthesis and protects cells from oxidative stress: relevance to Parkinson disease

The cause of selective dopaminergic neuronal degeneration in Parkinson disease has still not been resolved, but it has been hypothesized that oxidative stress and the ubiquitin-proteasome system are important in the pathogenesis. In this report, we investigated the effect of proteasome inhibition on oxidative stress-induced cytotoxicity in PC12 cells, an in vitro model of Parkinson disease. Treatment with proteasome inhibitors provided significant protection against toxicity by 6-hydroxydopamine and H(2)O(2) in a concentration-dependent manner. The measurement of intracellular reactive oxygen species using 2',7'-dichlorofluorescein diacetate demonstrated that lactacystin, a proteasome inhibitor, significantly reduced 6-hydroxydopamineand H(2)O(2)-induced reactive oxygen species production. Proteasome inhibitors elevated the amount of glutathione and phosphorylated p38 mitogen-activated protein kinase (MAPK) prior to glutathione elevation. The treatment with lactacystin induced the nuclear translocation of NF-E2-related factor 2 (Nrf2) and increased the level of mRNA for gamma-glutamylcysteine synthetase, a rate-limiting enzyme in glutathione synthesis. Furthermore, SB203580, an inhibitor of p38 MAPK, abolished glutathione elevation and cytoprotection by lactacystin. These data suggest that proteasome inhibition afforded cytoprotection against oxidative stress by the elevation of glutathione content, and its elevation was mediated by p38 MAPK phosphorylation.     

Redox proteomics: from residue modifications to putative biomarker identification by gel- and LC-MS-based approaches

Quantitative determination of reactive oxygen species and reactive nitrogen species in body fluids, tissues or cells has always been problematic due to their high chemical reactivity and the resulting short half-life. This high reactivity may involve reversible and/or irreversible protein modifications, in particular the covalent oxidative modification of specific amino acid residues. Thus, the occurrence of reactive oxygen species and reactive nitrogen species can be monitored indirectly from the identification of specific protein-chemical footprints. In combination with classical gel-based proteomics or liquid chromatography labeling or label-free techniques, mass spectrometry has emerged as a powerful tool to identify these protein modifications in biological samples. In this review, we present the main methodological approaches for gel-based proteomics and quantitative mass spectrometry applied to oxidative protein modifications, mainly Cys. Representative examples from their application in identifying respective biomarkers in diseases related to oxidative stress are also presented.     

Functional mitochondria are required for alpha-synuclein toxicity in aging yeast

alpha-Synuclein is one of the principal toxic triggers of Parkinson disease, an age-associated neurodegeneration. Using old yeast as a model of alpha-synuclein expression in post-mitotic cells, we show that alpha-synuclein toxicity depends on chronological aging and results in apoptosis as well as necrosis. Neither disruption of key components of the unfolded protein response nor deletion of proapoptotic key players (including the yeast caspase YCA1, the apoptosis-inducing factor AIF1, or the serine protease OMI) did prevent alpha-synuclein-induced cell killing. However, abrogation of mitochondrial DNA (rho(0)) inhibited alpha-synuclein-induced reactive oxygen species formation and subsequent apoptotic cell death. Thus, introducing an aging yeast model of alpha-synuclein toxicity, we demonstrate a strict requirement of functional mitochondria.     

Peroxidative aggregation of alpha-synuclein requires tyrosines

Alpha-synuclein is the main component of the intracellular protein aggregates in neurons of patients with Parkinson's disease. The occurrence of the disease is associated with oxidative damage. Although it is known that peroxidative chemistry leads to the aggregation of alpha-synuclein in vitro, the specific amino acid types of alpha-synuclein involved in this type of aggregation have not been identified. We show, using human cytochrome c plus H(2)O(2) as the source oxidative stress, that the tyrosines of alpha-synuclein are required for aggregation. The studies reveal the chemical basis for a crucial step in the aggregation process.     

The Role of Inflammatory and Oxidative Stress Mechanisms in the Pathogenesis of Parkinson’s Disease: Focus on Astrocytes

Neuroinflammation plays a key role in the pathogenesis of Parkinson’s disease (PD). Epidemiologic, animal, human, and therapeutic studies support the role of oxidative stress and inflammatory cascade in initiation and progression of PD. In Parkinson’s disease pathophysiology, activated glia affects neuronal injury and death through production of neurotoxic factors like glutamate, S100B, tumor necrosis factor alpha (TNF-α), prostaglandins, and reactive oxygen and nitrogen species. As disease progresses, inflammatory secretions engage neighboring cells, including astrocytes and endothelial cells, resulting in a vicious cycle of autocrine and paracrine amplification of inflammation leading to neurodegeneration. The exact mechanism of these inflammatory mediators in the disease progression is still poorly understood. In this review, we highlight and discuss the mechanisms of oxidative stress and inflammatory mediators by which they contribute to the disease progression. Particularly, we focus on the altered role of astroglial cells that presumably initiate and execute dopaminergic neurodegeneration in PD. In conclusion, we focus on the molecular mechanism of neurodegeneration, which contributes to the basic understanding of the role of neuroinflammation in PD pathophysiology.     

Fluorescence imaging of amyloid formation in living cells by a functional, tetracysteine-tagged alpha-synuclein

Alpha-synuclein is a major component of intraneuronal protein aggregates constituting a distinctive feature of Parkinson disease. To date, fluorescence imaging of dynamic processes leading to such amyloid deposits in living cells has not been feasible. To address this need, we generated a recombinant alpha-synuclein (alpha-synuclein-C4) bearing a tetracysteine target for fluorogenic biarsenical compounds. The biophysical, biochemical and aggregation properties of alpha-synuclein-C4 matched those of the wild-type protein in vitro and in living cells. We observed aggregation of alpha-synuclein-C4 transfected or microinjected into cells, particularly under oxidative stress conditions. Fluorescence resonance energy transfer (FRET) between FlAsH and ReAsH confirmed the close association of fibrillized alpha-synuclein-C4 molecules. Alpha-synuclein-C4 offers the means for directly probing amyloid formation and interactions of alpha-synuclein with other proteins in living cells, the response to cellular stress and screening drugs for Parkinson disease.     

Defective Autophagy in Parkinson’s Disease: Role of Oxidative Stress

Parkinson??s disease (PD) is a paradigmatic example of neurodegenerative disorder with a critical role of oxidative stress in its etiopathogenesis. Genetic susceptibility factors of PD, such as mutations in Parkin, PTEN-induced kinase 1, and DJ-1 as well as the exposure to pesticides and heavy metals, both contribute to altered redox balance and degeneration of dopaminergic neurons in the substantia nigra. Dysregulation of autophagy, a lysosomal-driven process of self degradation of cellular organelles and protein aggregates, is also implicated in PD and PD-related mutations, and environmental toxins deregulate autophagy. However, experimental evidence suggests a complex and ambiguous role of autophagy in PD since either impaired or abnormally upregulated autophagic flux has been shown to cause neuronal loss. Finally, it is generally believed that oxidative stress is a strong proautophagic stimulus. However, some evidence coming from neurobiology as well as from other fields indicate an inhibitory role of reactive oxygen species and reactive nitrogen species on the autophagic machinery. This review examines the scientific evidence supporting different concepts on how autophagy is dysregulated in PD and attempts to reconcile apparently contradictory views on the role of oxidative stress in autophagy regulation. The complex relationship between autophagy and oxidative stress is also considered in the context of the ongoing search for a novel PD therapy.     

Oxidative stress and neurodegeneration: the involvement of iron

Many evidences indicate that oxidative stress plays a significant role in a variety of human disease states, including neurodegenerative diseases. Iron is an essential metal for almost all living organisms due to its involvement in a large number of iron-containing proteins and enzymes, though it could be also toxic. Actually, free iron excess generates oxidative stress, particularly in brain, where anti-oxidative defences are relatively low. Its accumulation in specific regions is associated with pathogenesis in a variety of neurodegenerative diseases (i.e., Parkinson’s disease, Alzheimer’s disease, Huntington’s chorea, Amyotrophic Lateral Sclerosis and Neurodegeneration with Brain Iron Accumulation). Anyway, the extent of toxicity is dictated, in part, by the localization of the iron complex within the cell (cytosolic, lysosomal and mitochondrial), its biochemical form, i.e., ferritin or hemosiderin, as well as the ability of the cell to prevent the generation and propagation of free radical by the wide range of antioxidants and cytoprotective enzymes in the cell. Particularly, ferrous iron can act as a catalyst in the Fenton reaction that potentiates oxygen toxicity by generating a wide range of free radical species, including hydroxyl radicals (·OH). The observation that patients with neurodegenerative diseases show a dramatic increase in their brain iron content, correlated with the production of reactive oxigen species in these areas of the brain, conceivably suggests that disturbances in brain iron homeostasis may contribute to the pathogenesis of these disorders. The aim of this review is to describe the chemical features of iron in human beings and iron induced toxicity in neurodegenerative diseases. Furthermore, the attention is focused on metal chelating drugs therapeutic strategies.     

Shatavarin IV elicits lifespan extension and alleviates Parkinsonism in Caenorhabditis elegans

Shatavarin IV (SIV), a steroidal saponin, is a major bioactive phytomolecule present in roots of Asparagus racemosus (Liliaceae) known for its anticancer activity. Age-associated neurodegenerative Parkinson’s disease (PD) is characterised by alpha-synuclein aggregation in dopaminergic neuron resulting in neurodegeneration. The invention of bioactive molecules that delay aging and age-associated disorders endorses development of natural phytomolecule as a therapeutic agent for curing age-related diseases. Therefore, the present study for the first time explores the potential of SIV against aging and Parkinsonism utilising Caenorhabditis elegans model system. SIV significantly attenuated oxidative stress in terms of intracellular reactive oxygen species (ROS) as well as oxidative damage including protein carbonylation and also promotes longevity. SIV also significantly increased the mRNA expression of stress responsive genes namely sod-1, sod-2, sod-3, gst-4, gst-7 and ctl-2 suggesting its anti-oxidant property that might be contributed in the modulation of oxidative stress and promoting lifespan. Additionally, SIV improved PD symptoms by reducing the alpha-synuclein aggregation, lipid accumulation and enhancing dopamine level. Altogether, present findings indicate that SIV possibly utilising ubiquitin-mediated proteasomal system and attenuating oxidative stress by up-regulating PD-associated genes pdr-1, ubc-12 and pink-1. Therefore, this study is a forward step in exploring the anti-aging and anti-Parkinsonism potential of bioactive compound SIV in C. elegans.     

Single particle characterization of iron-induced pore-forming alpha-synuclein oligomers

Aggregation of alpha-synuclein is a key event in several neurodegenerative diseases, including Parkinson disease. Recent findings suggest that oligomers represent the principal toxic aggregate species. Using confocal single-molecule fluorescence techniques, such as scanning for intensely fluorescent targets (SIFT) and atomic force microscopy, we monitored alpha-synuclein oligomer formation at the single particle level. Organic solvents were used to trigger aggregation, which resulted in small oligomers ("intermediate I"). Under these conditions, Fe(3+) at low micromolar concentrations dramatically increased aggregation and induced formation of larger oligomers ("intermediate II"). Both oligomer species were on-pathway to amyloid fibrils and could seed amyloid formation. Notably, only Fe(3+)-induced oligomers were SDS-resistant and could form ion-permeable pores in a planar lipid bilayer, which were inhibited by the oligomer-specific A11 antibody. Moreover, baicalein and N'-benzylidene-benzohydrazide derivatives inhibited oligomer formation. Baicalein also inhibited alpha-synuclein-dependent toxicity in neuronal cells. Our results may provide a potential disease mechanism regarding the role of ferric iron and of toxic oligomer species in Parkinson diseases. Moreover, scanning for intensely fluorescent targets allows high throughput screening for aggregation inhibitors and may provide new approaches for drug development and therapy.     

Vascular oxidant stress: Molecular mechanisms and pathophysiological implications

The term oxidative stress refers to a situation in which cells are exposed to excessive levels of either molecular oxygen or chemical derivatives of oxygen (ie, reactive oxygen species). Three enzyme systems produce reactive oxygen species in the vascular wall: NADH/NADPH oxidase, xanthine oxidoreductase, and endothelial nitric oxide synthase. Among vascular reactive oxygen species superoxide anion plays a critical role in vascular biology because it is the source for many other reactive oxygen species and various vascular cell functions. It is currently thought that increases in oxidant stress, namely excessive production of superoxide anion, are involved in the pathophysiology of endothelial dysfunction that accompanies a number of cardiovascular risk factors including hypercholesterolemia, hypertension and cigarette smoking. On the other hand, vascular oxidant stress plays a pivotal role in the evolution of clinical conditions such as atherosclerosis, diabetes and heart failure.     

The neurotoxicity of iron,copper and cobalt in Parkinson’s disease through ROS-mediated mechanisms

Parkinson’s disease (PD) is the second most common neurodegenerative disease with gradual loss of dopaminergic neurons. Despite extensive research in the past decades, the etiology of PD remains elusive. Nevertheless, multiple lines of evidence suggest that oxidative stress is one of the common causes in the pathogenesis of PD. It has also been suggested that heavy metal-associated oxidative stress may be implicated in the etiology and pathogenesis of PD. Here we review the roles of redox metals, including iron, copper and cobalt, in PD. Iron is a highly reactive element and deregulation of iron homeostasis is accompanied by concomitant oxidation processes in PD. Copper is a key metal in cell division process, and it has been shown to have an important role in neurodegenerative diseases such as PD. Cobalt induces the generation of reactive oxygen species (ROS) and DNA damage in brain tissues.